German utility patent 80 29 580, shows a construction for adjusting the air gap of an electrically operable valve by means of narrowing down a flux gap through a closure opening depending on the magnetic excitation of the magnetic coil. As compared to standard magnetic values, this value has the advantage that it may be proportionally energized. It operates steadily and is constructed very simply compared to usual proportional valves.
In such valves, the use of a proportional magnet is not used and the characteristic line depends largely on the basic air gap of the magnetic circuit. This basic air gap is effected by assembly of various components having tolerances. For achieving a sufficiently precise air gap, it is therefore absolutely necessary that the tolerances obtained are compensated by shims. This makes assembly more difficult and the valve more expensive. Similar problems exist in the case of gap arrangements of cam valves or couplings. In such case adjustment for a certain basic air gap must be provided.